Motion picture film stock is composed of a photographic emulsion coated on a transparent plastic base. The plastic base and photographic emulsion are chemically unstable and are subject to decomposition and degradation over time. This is particularly true with respect to older film stock using cellulose nitrate, cellulose diacetate, or cellulose triacetate, which are known to be unstable mediums and suffer from known disadvantages including a low flash point, depolymerization of the film stock, vinegar syndrome, shrinkage, structural weakening, and brittleness. The photographic emulsion is also susceptible to damage from moisture or pressure, resulting in a distortion in the geometry of the film frame and stock. All of these conditions effectively degrade the condition of the original film strip, in some cases to the point where the film strip becomes unplayable.
While the original film can be repaired and cleaned, the degradation process cannot be stopped. Digital imagery is not subject to these degradation problems. It is therefore desirable to be able to accurately duplicate the film at a resolution equivalent to or greater than the original for archival purposes.
Standard specifications exist for 8 mm, 16 mm, 35 mm, and 70 mm wide film. These standards specify the height and width of the sprocket holes, the radius of the curves at the corners, the distances to the edge of the film, and the alignment tolerances between two sprocket holes and the edges. Prior to the adoption of standard specifications, sprocket holes varied widely in size and shape. It is desirable for a single machine to be able to handle a variety of film formats.
Older film stock presents a number of potential problems that need to be addressed for restoration. One such problem is film shrinkage, which is often irregular due to different levels of decay throughout the length of the film. Perforations or sprocket holes in films are also susceptible to damage from stress. Burrs or minor damage to film perforations can cause the film to be transported unsteadily and the resultant film image to appear to jump on screen. Film stock that has been spliced and repaired may also be damaged at the spliced location over time, resulting in complications due to uneven thickness or decaying adhesive.
Continuous motion telecine systems transport film by engaging sprocket teeth into the perforated holes that occur at even intervals along the edge of the film stock. Such equipment cannot readily handle transportation of film which may be shrunken, warped, or damaged. Such systems are generally incapable of handling film that has shrunken by more than a few percent, since the shrinkage causes the distance between perforations to become diminished and too closely spaced to engage a standard sprocket without the sprocket teeth potentially causing further damage to the perforations. Particularly with older film stock, the sprocket teeth in the telecine system may further damage the perforations along the edge of the film.
Digital scanning of each individual frame image requires a significant expenditure of time and expense. Instead of scanning frame by frame, some continuous motion telecine systems instead scan the film by storing an electronic video signal in a digital video format through a projector optically mated to a video camera. A disadvantage of these continuous motion telecine systems is that it is not possible to produce a picture from a still frame without having first moved the picture past the digital imaging device. Such systems are also incapable of producing individual digital image files representing an entire image area of each film being scanned at a high resolution. Still other systems are trigger image capture based on tachometric timing of the film being transported. Tachometer based systems are imprecise and result in variances in the vertical registration of successive image captures. Still other systems start and stop the transportation of the film on a frame by frame basis in order to capture each frame individually and to minimize issues arising from blurring or blending problems arising from the continuous motion of the film stock. The intermittent stop-start motion of the film stock in such systems results in additional stress to already endangered film stock and adds to time required to digitally archive a motion picture film.
A need therefore exists for a system that can create high resolution digital images of each individual frame of the film in an economic fashion. Such a machine should maintain a temporal correspondence between the generated video signal and the original motion picture film stock. Furthermore, the machine should be able to accept and safely handle film that may be shrunken, warped, brittle, or suffering other damage to the film stock or film perforations.